fix: DRM flip re-opens /scheme/drm/card0 instead of try_clone

Scheme files on Redox don't support try_clone(). Re-opening
the device node for each page flip is safe because DRM ioctls
are synchronous and the scheme serializes requests internally.

local/recipes/wayland/redbear-compositor/source/src/main.rs

@@ -163,11 +163,14 @@ mod drm_backend {
             let cur = self.current.load(Ordering::Relaxed);
             let next = (cur + 1) % self.fb_ids.len();
             let fb_id = self.fb_ids[next];
-            let mut f = self.file.try_clone().ok();
-            if let Some(ref mut f) = f {
-                let mut buf = Vec::with_capacity(12);
-                buf.extend_from_slice(&(DRM_IOCTL_MODE_PAGE_FLIP as u64).to_le_bytes());
-                buf.extend_from_slice(&fb_id.to_le_bytes());
+            // Page flip: write [u64_le ioctl][u32_le fb_id] to scheme
+            let mut buf = Vec::with_capacity(12);
+            buf.extend_from_slice(&(DRM_IOCTL_MODE_PAGE_FLIP as u64).to_le_bytes());
+            buf.extend_from_slice(&fb_id.to_le_bytes());
+            // Scheme I/O requires mutable access, so we re-open the device.
+            // This is safe because DRM ioctls are synchronous and the scheme
+            // serializes requests internally.
+            if let Ok(mut f) = File::open("/scheme/drm/card0") {
                 let _ = f.write_all(&buf);
             }
             self.current.store(next, Ordering::Relaxed);